Utilizing the Adaptive Polyoxometalate [As₂W₁₉O₆₇(H₂O)]^14– To Support a Polynuclear Lanthanoid-Based Single-Molecule Magnet

Abstract

Five members of a new family of polyoxometalate (POM)-ligated tetranuclear rare earth metal complexes have been synthesized and characterized. These compounds have the general formula (HDABCO)(8)H(5)Li(8)[Ln(4)As(5)W(40)O(144)(H(2)O)(10)(gly)(2)]·25H(2)O [Ln = Gd (1), Tb (2), Dy (3), Ho (4) and Y = (5), HDABCO = monoprotonated 1,4-diazabicyclooctane, gly = glycine] and were synthesized from the preformed POM precursor [As(2)W(19)O(67)(H(2)O)](14-). The structure is comprised of two {As(2)W(19)O(68)} building blocks linked by a unit containing four rare earth ions and two additional tungsten centers, with the two glycine ligands playing a key bridging role. Two crystallographically distinct rare earth ions are present in each complex, both of which possess axially compressed, approximate square antiprismatic coordination geometry. The variable-temperature magnetic susceptibility profiles for 2-4 are dominated by population/depopulation of the M(J) sublevels of the relevant ground terms, and fitting of the data has afforded the ligand field parameters in each case, from which the energies of the M(J) sublevels can be calculated. Alternating current magnetic susceptibility data have revealed the onset of slow magnetic relaxation for 3, with the energy barrier to magnetization reversal determined to be 3.9(1) K. As for other lanthanoid complexes that display slow magnetic relaxation, this energy barrier is due to the splitting of the M(J) sublevels of the Dy(3+) ions such that the ground sublevel has a relatively large |M(J)| value, thereby affording Ising-type magnetic anisotropy. This complex is thus the first POM-supported polynuclear lanthanoid-based SMM. Simulation of the W-band EPR spectrum of 1 has afforded the spin Hamiltonian parameters for this species, while the X-band EPR spectrum of 3 indicates the presence of a non-negligible fourth-order transverse component of the anisotropy, which is responsible for the small effective energy barrier observed for 3 and the absence of slow magnetic relaxation for 4.

First Tungstoantimonate-Based Transition-Metal–Lanthanide Heterometallic Hybrids Functionalized by Amino Acid Ligands

“…163,164 The χ M T reaches to the greatest value of 63.86 emu K mol −1 at 70 K during the cooling period (Figure 8c). This profile may …”

Polyoxometalates as Ligands for Functional Lanthanoid Complexes

“…35,36,45,49 The gadolinium complexes 1-Gd, 3-Gd and 6-Gd all display the spinonly magnetic behavior expected for gadolinium(III) ions (4f 7 , 8 S 7/2 ). Near perfect Curie-type susceptibility plots are observed with temperature-independent X m T behavior down to very low temperatures.…”

“…Overall the sodium templated polytungstate arrangement provides an open structure that accomodates four dysprosium centers. Three of the four dysprosium atoms are crystallographically unique with Dy1 and Dy3 displaying approximate square antiprismatic coordination geometries; meanwhile Dy2 has an approximately dodecahedral coordination 49 Similar to 4 and 5, 6 contains fragments originating from isomerization and decomposition of the tungstoarsenate precursor. The synthesis of 6 involved initial reaction followed by a very specific pH controlled recrystallization procedure incorporating lithium acetate.…”

Polyoxometalate-Supported Lanthanoid Single-Molecule Magnets

“…11). [47] Compounds (Ln ¼ Gd, Tb, Dy, and Ho) of series 10, are obtained following the addition of DABCO to an acidic aqueous solution of the POM precursor K 14 Unlike most of the other Ln-POM families discussed above, the four lanthanoid ions in 10 are not sandwiched between two lacunary POM ligands. However, as is observed for 1 and 2, the two distinct lanthanoid ions exhibit approximate trigonal dodecahedral and square antiprismatic coordination geometries.…”

“…13), which were performed using approximate Hamiltonians in Stevens notation (Eqn 1) for 1, 2, and [LnPc 2 ] À , and in Wybourne notation (Eqn 2) for 10. [12,36,47] The two formalisms are equivalent and the parameters can be easily converted from one into the other.…”